Included in the concept of energy absorption are the ability to absorb mechanical vibration, shock, and impulse such as with engine mounts and other shock absorbing devices, as well as absorb acoustical energy. By way of example only, the latter may include conventional sound waves as well as sonar at both sonic and ultrasonic frequencies. Energy absorption can occur in the form of mechanical transmission loss and echo reduction or insertion loss, for example in the field of sonic or ultrasonic sonar. Material is formulated such that the energy from mechanical or acoustic waves is attenuated therein, thereby reducing the energy in the wave as it traverses the material.
Energy-absorbing polymeric materials in general are known in the art. For example, U.S. Pat. No. 4,980,386 to Tiao et al. discloses polyurethane from polyol and polyisocyanate having shock-attenuating and low rebound attributes. U.S. Pat. No. 6,100,363 to Sampara et al. discloses polyurethane-based energy-absorbing elastomers comprising a water scavenger and exhibiting low resilience. U.S. Pat. No. 4,476,258 to Hiles discloses polyurethane elastomer compositions comprising a urethane-forming component and a diisocyanate exhibiting a compression set of less than about five percent and a recovery time of about 10 to about 100 milliseconds. Many materials, including that disclosed in U.S. Pat. No. 4,346,205 to Hiles, are composites which include, for example, hollow glass spheres (U.S. Pat. No. 4,079,162 to Metzger) or, gas bubbles as in a foam (U.S. Pat. No. 4,097,633 to Focht). Finally, perhaps the most well-known damper or isolator material is Sorbothane® from Sorbothane, Inc. of Kent, Ohio. Sorbothane® is a proprietary polyurethane which converts mechanical energy into heat.
Sonar-absorbing materials are also known in the art. U.S. Pat. No. 4,628,490 to Kramer et al. discloses a plurality of non-conducting elastomeric matrix layers having piezoelectric or magnetostrictive particles disposed therein. U.S. Pat. No. 5,444,668 to Sevik discloses an elastomeric matrix containing sealed air-filled cavities and random labyrinths of small water-filled passages. Finally, specialized syntactic foams for underwater acoustic applications (Syntactic Acoustic Damping Material—SADM) are offered by Syntech Materials, Inc. of Springfield, Va. The latter, however, contain lead, a potential environmental hazard.
Despite the wide variety of materials available for various energy-absorbent applications, there remain multiple needs for materials with improved properties which are, at the same time, less burdensome to the environment. For example, acoustic/sonar reduction materials which exhibit high insertion loss per inch, large and tailored echo reduction with relatively thin layers, tailored impedance, and mechanical integrity under deep ocean conditions can provide benefits not available with conventional materials. For mechanical applications, there is a need for materials which exhibit high internal loss and suitable mechanical properties such as hardness and toughness which may be tailored to the application for shock and vibration control.